1. Field of the Invention
The present invention relates to a data detecting apparatus and, more particularly, to a data detecting apparatus for detecting data of a reproduced signal coming from a channel used in digital magnetic recording apparatus or the like.
2. Description of the Prior Art
In Dolvio, F., "SIGNAL PROCESSING FOR HIGH DENSITY DIGITAL MAGNETIC RECORDING", CH2704-5/89/0000/1091/$01.00 c 1989, IEEE, 1-91 to 1-96, a data detecting apparatus was proposed, which detects reproduced signal data by observing a reproduced signal coming from a channel and decoding the signal by a Viterbi decoder. In the proposed constitution, it is necessary to give the Viterbi decoder two parameters; (1) a sample of a signal level value at a true data presence time (this is called "zero-degree phase") and (2) a mean value of zero-degree phase signal levels obtained by averaging noises contained in the reproduced signal.
It is therefore necessary for the data detecting apparatus to have both a phase-locked loop (hereinafter called a PLL) for making the apparatus be synchronized with zero-degree phase and a signal-level mean-value tracking circuit. Conventionally, these two functions have been implemented by controlling a sampling phase and a gain through a stochastic steepest descent algorithm.
With the above-mentioned conventional data detecting apparatus, however, jitters contained in an input reproduced signal causes a sampling phase to fluctuate, adversely affecting operations of the Viterbi decoder and digital circuits such as other decoders installed on subsequent stages. This disadvantage makes it difficult to design high-speed circuits in particular and perform tests on LSI circuits.